Electronic devices based on semiconductors such as diodes, transistors and integrated circuits can be found everywhere in our daily lives, in televisions, automobiles, household gadgets and computers. We have come to rely on them and increasingly have come to expect higher performance at a lower cost. Personal computers clearly illustrate this trend. A significant factor in the successful growth of the computer industry is that through technological advances, smaller and smaller transistors were fabricated and more of them could be incorporated in a given area. These devices deliver year after year better performance while consuming less power and because of their smaller size they can also be manufactured at a lower cost per device. For many of these applications, if the cost of the device is significantly reduced, many more applications can be envisaged. This is especially true for the photovoltaic or solar energy application, for a whole range of sensors as well as for other application areas such as ferroelectric devices, light emitting diodes for solid state lighting applications, storage applications such as computer hard disc drives, magnetoresistance based devices, photoluminescence based devices, non-volatile memory applications, dielectric devices, thermoelectric devices,
The use of renewable energy sources is essential for the future of the world we are living in. There is an unlimited potential for solar energy to power all the world's energy needs. However, for the past two decades, the promise of solar energy has remained unrealized. This is primarily because of the unfavorable price/performance metric of solar cells being manufactured today. In 1996, photovoltaics was a $1 Billion dollar per year business worldwide with more than 20 percent annual growth. Further increase in market share of the photovoltaic industry will require significant reductions in cost compared to most other forms of electricity generation typical in the utility industry. Higher efficiency cells made using more automated manufacturing processes are expected to lead the way to cost reduction. As prices decrease, usage of photovoltaic cells will increase, specially because of the environmental benefits offered by this technology compared to other forms of energy generation. Further technological innovations have the potential to cause the economic and commercial breakthrough necessary to lower prices to make solar energy cheaper than or equal to fossil fuels in cost.
Thin-film photovoltaics (PV) have a significant advantage over the traditional wafer-based crystalline Si cells. The primary advantage of thin films is cheaper materials and manufacturing costs and higher manufacturing yields compared to single-crystal technologies. Thin films use 1/20 to 1/100 of the material needed for crystalline Si PV and appear to be amenable to more automated, less expensive production. Currently, three film technologies are receiving significant interest from the industry for large scale PV: amorphous Si, CuInSe2 and CdTe. In most cases, module efficiencies are closely related to cell efficiencies, with minor losses (˜10%) due to some loss of active area and some electrical resistance losses. In order to further increase the efficiency and to be able to reproducibly fabricate thin-film based, high efficiency cells, microstructural features which limit the performance need to be controlled. While a complete understanding of the microstructural features which limit the performance are still unclear, it is reasonably well established that recombination at grain boundaries, intragrain defects and impurities is critical. In an effort to minimize the effect of grain boundaries, films with large grains are an objective.
Most thin-film solar cells are based on polycrystalline device layers since the cost of single crystal substrates is prohibitively expensive. Because they are polycrystalline, they do not have a well-defined crystallographic orientation (both out-of-plane and in-plane). Crystallographic orientation can have two important effects. The first is the effect of orientation of the growth surface on incorporation of dopants, intrinsic defects, and other impurities. Previous studies on a wide variety of dopants have shown that variations of 1 to 2 orders of magnitude can occur based on crystallographic orientation. An extreme effect of anisotropic doping is Si doping in GaAs films. Si doping in GaAs films, causes n-type conduction on (111) B-type GaAs, but p-type on (111) A-type GaAs. The second effect of crystallographic orientation is a variation in growth rate of the film being deposited. Both experiments as well as simulations have shown that under certain conditions growth rates can vary by 1 to 2 orders of magnitude as a function of crystallographic orientation. Uncontrolled crystallographic orientation in PV materials with large grain sizes may therefore result in reproducibility problems and hence lower yields during high volume production. Of course, grain boundaries at the intersection of grains in the polycrystalline film act as detrimental, recombination centers.
Most of the microstructural features currently thought to be limiting polycrystalline, thin-film, solar cell performance can be avoided by growing epitaxial films on lattice-matched, single crystal substrates. However, the high costs of single crystal substrates, prohibits their use for realistic applications. The effect of grain boundaries can be circumvented in polycrystalline photovoltaic thin films if the grain sizes are large enough (grain size at which effects on properties are minimal depend among other things on the doping level). However, in thin-films, grain growth is typically restricted to only twice the thickness of the film. Hence, grain boundaries in polycrystalline films have a dominant effect on efficiencies. A large number of studies have reported the effects of grain boundaries on photovoltaic properties.
While GaAs based solar cells need only be a micron or so thick to be effective. Silicon solar cells have to be much thicker, about 5-10 times thicker. Again as is the case with GaAs based solar cells, single crystal solar cells have the highest efficiencies of all. However, the high costs of fabricating single crystal Si or Ge wafers, prohibits the use of PV as an alternative energy source to fossil fuels. For Si-based solar cells, one would require a single crystal-like Si layer of greater than 10 μm, with an effective grain size of several hundred microns, preferably approaching a mm.
While much of the discussion above has focused on the solar cell application, there are numerous applications where a low cost, practically scalable method of fabricating single crystal-like semiconductor films is required where the effective size of the single crystal is at most a few cms in diameter. Furthermore, for certain applications, the semiconductor surfaces/films/wafers need to be flexible, thus enabling applications where a curved semiconductor may be desirable. For example, for a solar cell application it may be desirable to conform the PV module to the contour of a roof upon where it is placed. Thin-film transistors are used for fabricating displays. In this application one can also easily appreciate the use for a flexible and large-area displays.
Several techniques to fabricate single crystal-like substrates have recently been developed. These substrates have a surface which has a cubic crystal structure and is biaxially or triaxially textured. Triaxial texture in the substrate refers to situation when the three crystallographic axis of all the grains in a polycrystalline substrate are aligned within a certain angular range with respect to one another. The orientation of any grain in a material can be determined based on the orientation of the three crystallographic axis of the grain. These crystallographic axis are in general referred to as a, b, and c axis. For materials with higher crystal symmetry, such as cubic and tetragonal, these three axis are at right angles to one another or at 90 degrees with respect to one another. For such materials of high crystal symmetry, the alignment of any two axis necessarily implies the alignment of the third axis. Hence, description of the material as a biaxially textured material is adequate. However, for materials with lower symmetry such as monoclinic, the three axis are not at right angles to one another. In a polycrystalline sample of a complex monoclinic material, if the composition varies from grain to grain resulting in a change between the angles of the three crystallographic axis, then alignment of any two axis does not necessarily imply the alignment of the third. Hence, triaxial texture is the complete description of texture in such cases.
For cubic substrates used in the high temperature superconductor application, biaxial texture of sufficient quality for electromagnetic applications can be generally defined as being characterized by an x-ray diffraction phi scan peak of no more than 20° full-width-half-maximum (FWHM) and a omega-scan of 10° FWHM. The X-ray phi-scan and omega-scan measure the degree of in-plane and out-of-plane texture respectively. An example of a single-crystal-like texture is the cube texture with orientation {100}<100>, wherein the (100) crystallographic plane of all grains is parallel to the substrate surface and the [100] crystallographic direction is aligned along the substrate length. Another example of a single-crystal-like texture is the Goss texture with the orientation {110}<100>, wherein the (110) crystallographic plane of all grains is parallel to the substrate surface and the [100] crystallographic direction is aligned along the substrate length. Yet another example of a single-crystal-like texture is the orientation {210}<100>, wherein the (210) crystallographic plane of all grains is parallel to the substrate surface and the [100] crystallographic direction is aligned along the substrate length. Other suitable definitions have also been set forth in varying terms. Fabrication of single-crystal-like textured sheets or substrates via thermomechanical processing such as rolling and annealing is well established in the metallurgical and texture community worldwide. In addition to texturing via the thermomechanical processing route, there are other known routes to fabrication of biaxially textured, flexible electromagnetic devices known as ion-beam-assisted deposition (IBAD), inclined-substrate deposition (ISD) and deposition in the presence of a magnetic field. IBAD processes are described in U.S. Pat. Nos. 6,632,539, 6,214,772, 5,650,378, 5,872,080, 5,432,151, 6,361,598, 5,872,080, 6,190,752, 6,756,139, 6,884,527, 6,899,928, 6,921,741; ISD processes are described in U.S. Pat. Nos. 6,190,752 and 6,265,353; and biaxial texture by deposition in the presence of a magnetic field are described in U.S. Pat. No. 6,346,181; all these patents are incorporated herein by reference. Post-deposition ion-bombardment of uniaxially textured metal and/or alloy films on a unoriented, polycrystalline substrate can also result an in-plane texture via a selective grain growth process and in the extreme case the film can become triaxially textured. In all of these processes, a flexible, polycrystalline, untextured substrate or an amorphous substrate is used upon which buffer layers are deposited. One the key buffer layers is a biaxially textured layer that is deposited on this substrate using either IBAD, ISD or deposition in a magnetic field.
The following literature citations are incorporated by reference herein:
    1. NREL News release, NR-01296, May 10, (1996), http://www.nrel.gov/hotstuff/press/thinfilm.html.    2. Zweibel, K., “Thin Films: Past, Present, Future,” Progress in PV, The future of Thin Film Solar Cells, V. 3, # 5, 279-294, (1995).    3. Martin A. Green, Third Generation Photovoltaics, Advanced Solar Energy Conversion, Published by Springer-Verlag, ISSN 1437-0379, 2003.    4. Venkatasubramanian, R., O'Quinn, B. C., Hills, J. S., Sharps, P. R., Timmons, M. L., Hutchby J. A., Field, H., Ahrenkiel, R. and Keyes, B., “18.2% efficient GaAs Solar Cell on Optical-grade Polycrystalline Ge Substrates”, in Proceedings of the 25th IEEE Photovoltaic Specialists Conference, 1996, 31-36.    5. Bhat, R., Caneau, C., Zalt, C. E., Koza, M. A., Borner, W. A., Hwang, D. M., Schartz, S. A., Menocal S. G. and Favire, F. G., “Orientation dependence of S, Zn, Si, Te, and Sn doping in OMCVD growth of InP and GaAs—application to DH lasers and lateral p-n junction arrays grown on non-planar substrates,” J. Cryst. Growth, 107, 772-778 (1991).    6. Kondo, M., Anayama, C., Okada, N., Sekiguchi, H., Domen, K. and Tanabashi, T., “Crystallographic orientation dependence of impurity incorporation into III-IV compound semiconductors grown by metallorganic vapor phase epitaxy,” J. Appl. Phys., 76, 914-927 (1994).    7. Pavesi, I., Piazza, F., Hernioi, M and Harrison, I., Orientation dependence of the Si doping of GaAs grown by molecular beam epitaxy,” Semicond. Sci. and Tech., 8, 167-171 (1993).    8. Jones, S. H., Salinas, L. S., Jones, J. R. and Mayer, K, “Crystallographic orientation dependence of the growth rate for GaAs low pressure organometallic vapor phase epitaxy,” J. of Electron. Mater., 24, 5-14 (1995).    9. Salermo, J. P., Fan, C. C., McClelland, R. W., Vohl, P., Mavroides, J. G., Bozler, C. O., “Electronic Properties of Grain Boundaries in GaAs: A Study of Oriented Bicrystals Prepared by Epitaxial Lateral Overgrowth,” MIT Technical Report 669, May 10, 1984.    10. “Polycrystalline and Amorphous Thin Films and Devices”, edited by Kazmerski, L. L., Academic Press, New York, 1980.    11. “Grain Boundaries in Semiconductors,” edited by Leamy, H. J., Pike, G. E. and Seager, C. H., North-Holland, New York, 1982.    12. Card, H. C. and Yang E. S., “Electronic Processes at Grain Boundaries in Polycrystalline Semiconductors Under Optical Illumination,” IEEE Trans. Electron Devices, ED-24, 397 (1977).    13. Singh, R., Bhar, T. N., Shewchun, J. and Loferski, J. J., “Effect of Grain Boundaries on the Performance of Polycrystalline Tunnel MIS Solar Cells,” J. Vac. Sci. & Tech., 16, 236 (1979).    14. Kazmerski, L. L., “The Effects of Grain Boundaries and Interface recombination on the Performance of Thin-Film Solar Cells,” Solid-State Electron, 21, 1545 (1978); Kazmerski, L. L., Sheldon, P. and Ireland, P. J., Thin-Solid Films, 58, 95 (1979).    15. Goyal, A, Norton, D. P., Budai, J. D., Paranthaman, M., Specht, E. D., Kroeger, D. M., Christen, D. K., He, Q., Saffian, B., List, F. A., Lee, D. F., Martin, P. M., Klabunde, C. E., Hatfied, E. and Sikka, V. K., “High critical Current Density Tapes by Epitaxial Deposition of YBa2Cu3Ox Thick Films on Biaxially Textured Metals,” Appl. Phys. Lett., Sept., 69, 1795 (1996).    16. Silicon processing for the VSLI Era, Vol. 1, eds. S. Wolf and R. N. Tanber, pages 539-574, Lattice Press, Sunset Park, Calif., 1986.    17. Thesis S. D. and Wagner S., “Amorphous silicon thin-film transistors on steel foil substrates,” IEEE Electron Device Lett., vol. 17, no. 12, pp. 578-580, December 1996.    18. Serikawa T. and Omata F., “High-mobility poly-Si TFT's fabricated on flexible stainless steel substrates,” IEEE Electron Device Lett., vol. 20, no. 11, pp. 574-576, November 1999.    19. Afentakis T. and Hatalis M., “High performance polysilicon circuits on thin metal foils,” Proc. SPIE, vol. 5004, pp. 122-126, 2003.    20. Howell R. S., Stewart M., Karnik S. V., Saha S. K. and Hatalis M. K., IEEE Electron Device Lett., vol. 21, no. 2, pp. 70-72, February 2000.    21. Afentakis T., Hatalis M., Voutsas T. and Hartzell J., “Poly-silicon TFT AM-OLED on thin flexible metal substrates,” Proc. SPIE, vol. 5004, pp. 187-191, 2003.    22. Martin A. Green, Keith Emery, Klaus Bücher, David L. King, Sanekazu Igari, “Solar cell efficiency tables (version 11),” Progress in Photovoltaics: Research and Applications, Volume 6, Issue 1, Pages 35-42, 4 May 1999.    23. Karam, N. H.; King, R. R.; Cavicchi, B. T.; Krut, D. D.; Ermer, J. H.; Haddad, M.; Li Cai; Joslin, D. E.; Takahashi, M.; Eldredge, J. W.; Nishikawa, W. T.; Lillington, D. R.; Keyes, B. M.; Ahrenkiel, R. K., “Development and characterization of high-efficiency Ga0.5In0.5P/GaAs/Ge dual- and triple-junction solar cells,” Electron Devices, IEEE Transactions on, Vol. 46, No. 10, pp. 2116-2125, October 1999.    24. H. Hou, K. Reinhardt, S. Kurtz, J. Gee, A. Allerman, B. Hammons, P. Chang, E. Jones, Novel InGaAsN pn junction for high-efficiency multiple-junction solar cells, The Second World Conference on PV Energy Conversion, 1998, pp. 3600-3603.    25. D. Friedman, J. Geisz, S. Kurtz, J. Olson, 1-eV GaInNAs solar cells for ultra high efficiency multijunction devices, The Second World Conference on PV Energy Conversion, 1998, pp. 3-7; T. V. Torchynska and G. Polupan, “High efficiency solar cells for space applications,” Superficies y Vacío 17(3), 21-25, septiembre de 2004.    26. R. McConnell and M. Symko-Davies, “DOE High Performance Concentrator PV Project,” International Conference on Solar Concentrators for the Generation of Electricity or Hydrogen, 1-5 May 2005, Scottsdale, Ariz., NREL/CD-520-38172.    27. Michelle J. McCann, Kylie R. Catchpole, Klaus J. Weber, Andrew W. Blakers, “A review of thin-film crystalline silicon for solar cell applications. Part 1: Native substrates,” Solar Energy Materials and Solar Cells, Vol. 68, Issue 2, May 2001, Pages 135-171.    28. Kylie R. Catchpole, Michelle J. McCann, Klaus J. Weber and Andrew W. Blakers, “A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates,” Solar Energy Materials and Solar Cells, Vol. 68, Issue 2, May 2001, Pages 173-215.    29. B. Cunningham, H. Strunk and D. G. Ast, “First and second order twin boundaries in edge defined film growth silicon ribbon, Appl. Phys. Lett., 40, pp. 237-239, 982    30. M. Rinio, M. Kaes, G. Hahn and D. Borchert, “Hydrogen passivation of extended defects in multicrystalline silicon solar cells,” Presented at the 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, Germany, 4-8, 9, 2006.    31. A. Ebong, M. Hilali, A. Rohtagi, D. Meier and D. S. Ruby, “Belt furnace gettering and passivation of n-web silicon for high-efficiency screen-printed front-surface field solar cells,” Progress in Photovoltaics: Research and Applications, 9, pp. 327-332, 2001.    32. C. H. Seager, D. J. Sharp and J. K. G. Panitz, “Passivation of grain boundaries in silicon,” J. Vac. Sci. & tech., 20, pp. 430-435, 1982; N. H. Nickel, N. M. Johnson and W. B. Jackson, “Hydrigen passivation of grain boundary defects in polycrystalline silicon thin films,” Appl. Phys. Lett., 62, pp. 3285-3287, 1993.    33. A. Ashok, “Research in hydrogen passivation of defects and impurities in silicon,” NREL Report No. NREL/SR-520-36096, May 2004; M. Lipinski, P. Panek, S. Kluska, P. Zieba, A. Szyszka and B. Paszkiewicz, “Defect passivation of multicrystalline silicon solar cells by silicon nitride coatings,” Materials Science-Poland, vol. 24, pp. 1003-1007, 2006.    34. V. Yelundur, “Understanding and implementation of hydrogen passivation of defects in string ribbon silicon for high-efficiency, manufacturable, silicon solar cells,” Ph.D. thesis, Georgia Institute of Technology, Atlanta, Ga., November 2003.    35. R. Hühne, S. Fähler, B. Holzapfel, “Thin biaxially textured TiN films on amorphous substrates prepared by ion-beam assisted pulsed laser deposition,” Appl. Phys. Lett., vol. 85, pp. 2744-2746, 2004.    36. Jin-Hyo Boo 1, S. A. Ustin, W. Ho, “Supersonic jet epitaxy of single crystalline cubic SiC thin films on Si substrates from t-Butyidimethylsilane,” Thin Solid Films 324, pp. 124-128, 1988.    37. Lars Oberbeck, Jan Schmidt, Thomas A. Wagner and Ralf B. Bergman, “High rate deposition of epitaxial layers for efficient low-temperature thin film epitaxial silicon solar cells,” Progress in Photovoltaics: Research and Applications, vol. 9, pp. 333-340, 2001.    38. J. Carabe and J. J. Gandia, “Thin-film-silicon Solar Cells,” OPTO-Electronics Review, vol. 12, pp. 1-6, 2004; S Summers, H S Reehal and G H Shirkoohi, “The effects of varying plasma parameters on silicon thin film growth by ECR plasma CVD,” J. Phys. D: Appl. Phys. Vol. 34, pp. 2782-2791, 2001.    39. Thomas A. Wagner, Ph.D. thesis, “Low temperature silicon epitaxy: Defects and electronic properties,” Institut fur Physikalische Elektronik der Universit at Stuttgart, 2003.    40. Hjemas, P. C., Lohne, O., Wandera, A., Tathgar, H. S., “The effect of grain orientations on the efficiency of multicrystalline solar cells,” Solid State Phenomena, vol. 95-96, pp. 217-222, 2004.    41. Liwen tan, Qiyuan Wang, Jun Wang, Yuanhuan Yu, Zhongli Liu and Lanying Lin, “Fabrication of novel double-hetero-epitaxial SOI structure Si/γ-Al2O3/Si,” Journal of Crystal Growth, vol. 247, pp. 255-260, 2003.    42. K. Sawada, M. Ishida, T. Nakamura and N. Ohtake, “Metalorganic moelecular beam epitaxy of films on si at low growth temperatures,” Appl. Phys. Lett., vol. 52, pp. 1672-1674, 1988.    43. M. Shahjahan, Y. Koji, K. Sawada and M. Ishida, “Fabrication of resonance tunnel diode by gamma-Al2O3/Si multiple heterostructures,” Japn. J. of Appl. Phys. Part 1, vol. 41 (4B), pp. 2602-2605, 2002.    44. Hattangady, S. V., Posthill, J. B., Fountain, G. G., Rudder R. A., Mantini and M. J., Markunas, R. J., “Epitaxial silicon deposition at 300° C. with remote plasma processing using SiH4/H2 mixtures,” Appl. Phys. Lett., vol. 59(3), pp. 339-341, 1991.    45. Wagner, T. A., Oberbeck, L., and Bergmann, R. B., “Low temperature epitaxial silicon films deposited by ion-assisted deposition,” Materials Science & Engineering B-Solid State Materials for Advanced Technology, vol. 89, pp. 1-3, 2002.    46. Overbeck, L., Schmidt, J., Wagner, T. A., and Bergmann R. B., “High-rate deposition of epitaxial layers for efficient low-temperature thin film epitaxial silicon solar cells,” Progress in Photovoltaics, vol. 9(5), pp. 333-340, 2001.    47. Thiesen, J., Iwaniczko, E., Jones, K. M., Mahan, A., and Crandall, R., “Growth of epitaxial silicon at low temperatures using hot-wire chemical vapor deposition,” Appl. Phys. Lett., vol. 75(7), pp. 992-994, 1999.    48. Ohmi, T., Hashimoto, K., Morita, M., Shibata, T., “Study on further reducing the epitaxial silicon temperature down to 250° C. in low-energy bias sputtering,” Journal of Appl. Phys., vol. 69(4), pp. 2062-2071, 1991.    49. Cline H. E., “A single crystal silicon thin-film formed by secondary recrystallization,” Journal of Appl. Phys., vol. 55 (12), pp. 4392-4397, 1984.    50. R. Chowdhury, X. Chen and J. Narayan, “Pulsed laser deposition of Si/TiN/Si(100) heterostructures,” Appl. Phys. Left., vol. 64, pp. 1236-1239, 1994.    51. Santos, P. V.; Trampert, A.; Dondeo, F.; Comedi, D.; Zhu, H. J.; Ploog, K. H.; Zanatta, A. R.; Chambouleyron, I. “Epitaxial pulsed laser crystallization of amorphous germanium on GaAs,” Journal of Applied Physics, Vol. 90, pp. 2575-2581, 2001.    52. T. Sameshima, H. Watakabe, H. Kanno, T. Sadoh and M. Miyao, “Pulsed laser crystallization of silicon-germanium films,” Thin Solid Films Vol. 487 pp. 67-71, 2005.    53. Jin-Hyo Boo, S. A. Ustin and W. Ho, “Supersonic jet epitaxy of single crystalline cubic SiC thin films on Si substrates from t-Butyidimethylsilane,” Thin solid Films, vol. 324, pp. 124-128, 1998.    54. K. Eisenbeiser, R. Emrick, R. Droopad, Z. Yu, J. Finder, S. Rockwell, J. Holmes, C. Overgaard, and W. Ooms, “GaAs MESFETs Fabricated on Si Substrates Using a SrTiO3 Buffer Layer,” IEEE Electron Device Letters, Vol. 23, No. 6, pp. 300-302, 2002.    55. Droopad R, Yu ZY, Li H, Liang Y, Overgaard C, Demkov A, Zhang XD, Moore K, Eisenbeiser K, Hu M, Curless J, Finder J, “Development of integrated hetero structures on silicon by MBE,” Journal of Crystal Growth, vol. 251 (1-4), pp. 638-644, 2003.    56. R. D. Ott, P. Kadolkar, C. A. Blue, A. C. Cole, and G. B. Thompson, “The Pulse Thermal Processing of Nanocrystalline Silicon Thin-Films,” JOM, vol. 56, pp. 45-47, October, 2004.    57. D. Cahen and R. Noufi, “Defect chemical explanation for the effect of air anneal on CdS/CuInSe2 solar cell performance,” Appl. Phys. Left., vol. 54, pp. 558-560, 1989).    58. L. Kronik, D. Cahen, and H. W. Schock, “Effects of Sodium on Polycrystalline Cu(In,Ga)Se2 and Its Solar Cell Performance,” Advanced Materials, vol. 10, pp. 31-36, 1999.    59. M. J. Romero, K. Ramanathan, M. A. Contreras, M. M. Al-Jassim, R. Noufi, and P. Sheldon, “Cathodoluminescence of Cu(In,Ga)Se2 thin films used in high-efficiency solar cells,” Appl. Phys. Left., vol. 83, pp. 4770-4772, 2003.    60. Persson C, Zunger A., “Anomalous grain boundary physics in polycrystalline CuInSe2: the existence of a hole barrier,” Phys. Rev. Left. vol. 91, pp. 266401-266406, 2003.    61. C.-S. Jiang, R. Noufi, K. Ramanathan, J. A. AbuShama, H. R. Moutinho, and M. M. Al-Jassim, “Local Built-in Potential on Grain Boundary of Cu(In,Ga)Se2 Thin Films,” Conference Paper, NREL/CP-520-36981, 2005.    62. M. J. Hetzer, Y. M. Strzhemechny, M. Gao, M. A. Contreras, A. Zunger, and L. J. Brillson, “Direct observation of copper depletion and potential changes at copper indium gallium diselenide grain boundaries,” Appl. Phys. Left. vol. 86, pp. 162105-162107, 2005.    63. A. Ohno, “Grain growth control by solidification technology,” Materials Science Forum, vol. 204-206, pp. 169-178, 1996.    64. S. Chaisitsak, A. Yamada and M. Konagai, “Preferred Orientation Control of Cu(In1-xGax)Se2 (x≈0.28) Thin Films and Its Influence on Solar Cell Characteristics,” Jpn. J. Appl. Phys. vol. 41, pp. 507-513, 2002.    65. Olliges S, Gruber P, Bardill A, Ehrler D, Carstanjen HD and Spolenak R, “Converting polycrystals into single crystals—Selective grain growth by high-energy ion bombardment,” Acta Meterialia, vol. 54, pp. 5393-5399.    66. Dinescu M, Stanciu C, Ghica D, Dinu R, Sandu V, Nastase N, Balucani M, Bondarenko V, Frachina L, Lamedica G, Ferrari A, “Multilayer structures deposited by laser ablation,” Sensors and Actuators, vol. 74, pp. 27-30, 1999.    67. J. M. E. Harper, J. Gupta, D. A. Smith, J. W. Chang, K. L. Holloway, D. P. Tracey and D. B. Knorr, “Crystallographic texture change during abnormal grain growth in Cu—Co thin films,” Appl. Phys. Lettl, vol. 65, pp. 177-179, 1994.    68. T. Ohmi, T. Saito, M. Otsuki, T. Shibuta and T. Nitta, “Formation of copper thin films by a low kinetic energy particle process,” J. of Electrochemical Soc., vol. 138, pp. 1089-1097, 1991